1. Field of the Invention
The invention concerns a soda-lime glass containing vanadium, in particular a flat glass, produced by the float process with high UV absorption for wavelengths below 350 nm.
2. Background Information
Glasses with high UV absorption, high UV absorption being taken here to mean UV absorption up to about 90% of the UV radiation at wavelengths of below 400 nm, have until now been used only as flat glasses in isolated cases, preferably in the glass container industry for the manufacture of glass containers, such as bottles, in which UV-sensitive foodstuffs, such as, for example, milk, yogurt, juices containing vitamins or the like, are transported and stored.
For some time now, there has been considerable interest in flat glasses highly absorbent in the UV region, with high level of transmittance in the visible region, which are essentially neutral in color. By high level of transmittance is meant here the minimum transmittance values for nearly normal incidence for flat glass in building with planar parallel surfaces and different thickness are given by the following Table 1:
TABLE 1 ______________________________________ Light transmittance and solar radiation transmittance for non-colored glasses with parallel flat surfaces thickness light transmittance radiation transmittance mm Value .tau. min. Value .tau..sub.e min. ______________________________________ 3 0.88 0.83 4 0.87 0.80 5 0.86 0.77 6 0.85 0.75 8 0.83 0.70 10 0.81 0.65 12 0.79 0.61 15 0.76 0.55 19 0.72 0.48 ______________________________________
This interest concerns both glasses for the construction and the automotive sector, for example, in order to protect shop window displays from fading or also to protect plastic automotive interior materials from decomposition, premature aging and in particular, as well from color changes. A particular application for the use of UV-absorbent flat glasses is found in the manufacture of laminated glass window panes, in particular those with cast resin or fire protection interlayers.
These interlayers or the adhesives used therewith must at present still be provided with costly UV-resistant constituents, or, if this is not the case, it is also customary particularly in the case of waterglass-based fire protection interlayers, to provide UV-absorbent layers in a glass sandwich or to use these fire-retardant laminated glass panes only in building interiors, where no UV rays occur. Particularly UV-sensitive, for example, is glycerine, which is used inter alia in fire protection waterglass layers.
UV-absorbent soda-lime glasses have been described, for example, in JP-A 52-47 811, in which 0.035 to 0.12 weight % V.sub.2 O.sub.5 have been used jointly with 0.006 to 0.08 weight % manganese oxide and less than 0.4 ppm Co.sub.2 O.sub.3. This soda-lime glass does possess UV-absorption capability, but for color correction required--apart from iron--the use of three color-changing substances. In addition, this is a glass mixture which contains MnO.sub.2. Brownstone (MnO.sub.2) cannot be used without problems under the process conditions of float glass manufacture, as for stabilization of Mn.sup.3+ in the molten glass, pronouncedly oxidizing melting conditions are necessary, which can be achieved only with difficulty in the melting end of a floating plant. At least partial reduction of the manganese to form practically colorless Mn.sup.2+ can thus hardly be avoided with the float process.
In addition, it is known that in the course of time glasses containing Mn.sup.2+ "solarize", i.e., redox reactions are triggered by UV light, during which, due, for example, to Fe.sup.3+ ions present in the glass--or also due to other oxidizing constituents--the color less Mn.sup.2+ is oxidized to form violet-tinting Mn.sup.3+. This means that the hue of glasses containing Mn.sup.2+ gradually changes under the influence of UV light.
As an alternative to MnO.sub.2, selenium oxide is used, but selenium oxide is to be avoided on account of its environmental pollution and toxicity. In addition, selenium has a tendency to sublimate, so that it is necessary to implement extensive emission control measures. In JP-A 59-50 045, a soda-lime glass with high UV and IR absorption has been proposed, which used 0.05 to 1.0 weight % V.sub.2 O.sub.5, 0.2 to 0.5 Fe.sub.2 O.sub.3 and 0 to 5 weight % TiO.sub.2 in a glass compound as coloring component, whereby this glass possesses at least 50% transmittance for radiation in the visible region and up to 75% transmittance for solar radiation. This glass absorbs radiation below 370 nm almost completely, as a result of which fading and photoreactions of articles protected by this glass can be prevented. A disadvantage of this glass lies in the fact that its transmittance is adequate only for radiation in the visible region. The teaching given there for production of a UV-absorbent float glass with high transmittance in the visible region, which is essentially neutral in color, is insufficient however.
From GB-PS 708 031, a soda-lime glass (crown glass) with a refractive index of about 1,523 is known, this can be used as optical glass or glass with special refractive properties and has been designed for the production of spectacle glasses and the like. In the case of this application, UV absorption is desirable for protection of the eye. This glass possesses no UV transmittance at least 75%. To achieve these properties, 0.2 to 1.2 weight % vanadium oxide together with at least one additional color modifying metal oxide, selected from the group of iron, cobalt, copper, chromium, tin, arsenic, antimony, manganese, as well as other constituents, has been used.
This glass does in fact possess good UV-absorbent properties with simultaneously satisfactory transmittance in the visible region; the glass is not, however, suitable for manufacture of flat glasses, in particular by the floating process, as it does not possess the viscosity and floating properties necessary for the float glass process. The manganese oxide proposed for color matching cannot be employed in the float glass process for the reasons stated above.
In addition, UV-absorbent borosilicate glasses are known from EP-C 0 321 297, which possess a combination of color-changing additives, selected from the group consisting of cerium oxide, manganese oxide, iron oxide, cobalt oxide, copper oxide, vanadium oxide, molybdenum oxide. The glass proposed there, particularly for the manufacture of automotive windshields is costly to produce, as the boron materials to be used are expensive. These tend to volatilize and form environmentally pollutant emissions which, on account of the high cost involved in eliminating these emissions, should be avoided. EP-C 0321 297 does not provide a precise recommendation as to how vanadium oxide can be combined with other metal oxides to prevent UV transmittance; it is recommended that the costly oxides of the rare earths, for example, cerium or even lanthanum should be used to improve the absorption behavior, so that the use of vanadium oxides is stated solely as an optional additional possibility, but without any information as to the quantity it is used jointly with other pigments. Float glass compounds cannot be produced by this means.
DE-C 20 06 078 concerns sheet glasses to reduce transmittance in the visible and infrared spectral region, which incorporate a mixture of copper oxide, vanadium pentoxide, ferric oxide, nickel oxide and cobalt oxide, whereby the sum of these colorific components, whose proportion in relation to one another can be varied, is at least 2.9 weight % of the glass mixture. The special high-flatness glass which can be produced by this method is too cost intensive for the manufacture of float glass and is developed here for the special application of production of magnetic/optical storage plates. Naturally, little importance is attached to maximum neutrality of color.
A UV-absorbent display window pane is known from DE-C 916 217, whereby for absorption of short wave radiation below about 400 nm it is proposed that about 2 weight % cerium or titanium oxide be added to the normal display window glass composition.
From DE-C 14 96 072, a filter glass with a very high silicic acid content of 96 weight % is known, where pronounced UV absorption is achieved by means of V.sub.2 O.sub.5 as an additive together with cerium nitrate and titanium dioxide. The high silicate content glasses cannot be processed by the float glass method.
In U.S. Pat. No. 2,860,059, a UV-absorbent glass is described which has a cerium oxide content of 0.05 to 0.5 weight % CeO.sub.2 and 0.2 to 0.6 weight % Fe.sub.2 O.sub.3. This glass is expensive to manufacture on account of the high cerium content. The high iron content leads to unfavorable transmittance values.
Accordingly, UV-absorbent glasses of widely differing types have heretofore been known, but as yet no UV-absorbent glasses with high transmittance in the visible region, which are suitable for the cost effective manufacture of float glass in large quantities.
In this connection, the following is a brief description of the preferred flat glass production process, which represents the state of the art.
With the float glass process, the molten glass is passed to a floating bath of molten tin, on which it floats. As protection against oxidation of the tin, the process is carried out in a reducing inert gas atmosphere.
On account of this reducing atmosphere, the use of various additives, which can be employed in conventional flat glass not produced by the float glass process, is not possible. Thus, it is not permissible to use lead, nickel or copper oxides or similar oxides as additives, as these will be reduced to the metal on the surface of the glass ribbon. The use of aluminum oxide to ensure the hydrolytic stability of the glass and also to control the viscosity of the molten glass and the use of sodium oxide for liquefaction of the molten glass is also important, so that only very precisely adjusted glasses can be used. The tin bath has a temperature gradient of 600.degree. to 1100.degree. C., whereby at the end of it at 600.degree. C., the floating glass ribbon is lifted off and slowly cooled.